Re: [PATCHv3 2/2] arch: Rename CONFIG_DEBUG_RODATA and CONFIG_DEBUG_MODULE_RONX
From: Ingo Molnar
Date: Mon Feb 20 2017 - 02:24:24 EST
* Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> On Thu, Feb 16, 2017 at 2:25 PM, Pavel Machek <pavel@xxxxxx> wrote:
> > Hi!
> >> -config DEBUG_RODATA
> >> +config STRICT_KERNEL_RWX
> >> bool "Make kernel text and rodata read-only" if ARCH_OPTIONAL_KERNEL_RWX
> >> depends on ARCH_HAS_STRICT_KERNEL_RWX
> >> default !ARCH_OPTIONAL_KERNEL_RWX ||
> > Debug features are expected to have runtime cost, so kconfig help is
> > silent about those. But there are runtime costs, right? It would be
> > nice to mention them in the help text...
> It depends on the architecture. The prior help text for arm said:
> The tradeoff is that each region is padded to section-size (1MiB)
> boundaries (because their permissions are different and splitting
> the 1M pages into 4K ones causes TLB performance problems), which
> can waste memory.
> parisc (somewhat inaccurately) said:
> This option may have a slight performance impact because a
> portion of the kernel code won't be covered by a TLB anymore.
> IIUC, arm64 does what parisc is hinting at: mappings at the end are
> broken down to PAGE_SIZE. On x86, IIUC, there's actually no change to
> TLB performance due to how the mappings are already set up.
BTW., a good strategy with RAM sizes above say 4GB would be to just round up to
the next large-TLB boundary (2MB) and waste 0-2MB of RAM - which is only 0.05% of
4GB of RAM. On most workloads, especially with SSDs it's probably a positive RAM
vs. performance trade-off.